Paleostress history estimated from preferred orientation of microcracks in the Tanzawa tonalite body, central Japan and its relationship to the Honshu arc-Izu arc collision

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  • Sato Takatsune
    Department of Earth Sciences and Environmental and Resources Engineering, Graduate School of Science and Engineering, Waseda University Present address: Materials Technology Division, Railway Technical Research Institute
  • Takagi Hideo
    Department of Earth Sciences, Faculty of Education and Integrated Arts and Sciences, Waseda University

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Other Title
  • 丹沢トーナル岩体における石英中のマイクロクラックから推定された古応力場の変遷と伊豆弧の衝突
  • タンザワ トーナルガンタイ ニ オケル セキエイ チュウ ノ マイクロクラック カラ スイテイ サレタ コ オウリョクバ ノ ヘンセン ト イズ コ ノ ショウトツ

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This paper discusses the paleostress field and its history in the Tanzawa tonalite body in the Izu arc-Honshu arc collision zone, as determined from the 3-D orientation of healed and sealed intracrystalline microcracks in quartz grains, in conjunction with microthermometry of fluid inclusions in healed microcracks. Crosscutting relationships indicate that the healed microcracks formed prior to the formation of sealed (and possibly open) microcracks. Both healed and sealed microcracks strike NNE-SSW and dip vertically. It is estimated that healed microcracks were formed at a pressure range of 0.20-0.29 GPa and a temperature range of 275-410°C, based on isochores inferred from microthermometry of the fluid inclusions and with reference to the geothermal gradient (30-50°C/km). The formation temperature of the healed microcracks can be correlated with the closure temperature of the biotite K-Ar system, considering that the Azegamaru and Yushin plutons of the Tanzawa tonalite body yield K-Ar biotite ages of 5-4 Ma. The healed microcracks probably formed in the early Pliocene during collision of the Tanzawa block against the Honshu arc. Because the dominantly NNE-SSW trending microcracks were rotated clockwise by 10° during the collision of the Izu block against the Honshu arc at about 1 Ma, as deduced from reported paleomagnetic data, the orientation of maximum horizontal stress (σHmax) in the Tanzawa tonalite body during and after its collision is inferred to have trended nearly N-S. Recently reported in situ stress measurements in the Tanzawa tonalite body indicate that σHmax trends NNE-SSW. Accordingly, the N-S to NNE-SSW trends in σHmax have not changed greatly during or after the formation of healed microcracks in the Tanzawa tonalite body, which occurred at about 5-4 Ma.

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